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PRINCESS LEIA. Animation supervisors on 'Rogue One: A Star Wars Story' share the process of putting characters such as a young Princess Leia and Death Star commander Grand Moff Tarkin back on film. Screengrab from YouTube/Star Wars

LOS ANGELES, USA – It grossed $1 billion and picked up numerous awards nominations, but a particular aspect of Rogue One: A Star Wars Story proved one of the most controversial moments in the entire franchise.

With Lucasfilm planning to release the DVD and Blu-ray of Rogue One on April 4, fans will get an insight into just how well the film's crew understood that its flirtation with the "uncanny valley" of computer-generated human images was a huge risk.

In a bonus featurette entitled "The Princess and The Governor," animation supervisor Hal Hickel discusses a cutting-edge special effects process he describes as a "long series of failures resulting in victory."

"There were many dark days, many sleepless nights, laying awake, worrying about these shots," he says.

Developed by a Japanese robotics professor in 1970, the "uncanny valley" is the hypothesis that human replicas that appear almost, but not quite, like real humans elicit feelings of revulsion.

Its name refers to the sudden dip in our emotional response, which generally grows more positive the more human the replicas look – until they are so life-like that we are creeped out.

"Close up digital human works is one of the hardest problems in computer graphics," visual effects supervisor John Knoll explains on the featurette.

"You don't want to be sitting there in the theater saying 'Yeah, something doesn't look right. What do you think that is?'"

'Unnerving'

Unfortunately, this was the exact reaction of numerous otherwise rapt critics who thought the movie's perilous deep dive into the "uncanny valley" had undermined its many positive qualities.

Cushing, who played villainous Death Star commander Grand Moff Tarkin in the original film, died in 1994, while Fisher had stopped looking like 19-year-old Leia decades before her untimely death just two weeks after Rogue One came out.

So the idea of creating CGI versions of the actors was hugely divisive, with The Washington Times's Eric Althoff dismissing their inclusion as "effing weird."

Kelly Lawler of USA Today complained that while Tarkin was "unnerving," the Leia cameo was "so jarring as to take the audience completely out of the film at its most emotional moment."

Tarkin and Leia are played by Guy Henry and Ingvild Deila, with the digital likenesses of the original actors superimposed by San Francisco-based effects studio Industrial Light and Magic (ILM).

"It takes a lot of preparation to get into this character because everyone remembers Leia very well, so it needs to look exactly right," Deila explains on the featurette.

"And so they spent a lot of time on my hair, obviously. They dyed it twice and then added some extra hair... in the front because her hairline is a bit lower than mine. And also a big chunk of hair to make the buns.

"Then all these dots were put on right before we started shooting so that they could put Carrie Fisher's face on top of mine."

'Carrie loved it'

While archived audio is mined for Leia's sole word of dialogue, Henry comes up with a passable Cushing impersonation for his substantial scenes of dialogue.

"It was a very difficult process because there's no way to really go back in time and capture the appearances of these actors," said ILM creative director Paul Giacoppo.

"So we had to really bring every single possible skill set to bear, to try to recreate the details of their facial appearance and skin likeness and performance."

Lucasfilm president Kathleen Kennedy, a producer on the movie, says she wouldn't have green-lit the digital resurrections if visual effects supremo John Knoll hadn't been so confident he could pull it off.

As for Fisher herself, the actress managed to see her cameo before her death last December 27 at the age of 60, Knoll told ABC News, and gave it her blessing.

"She was involved in the process and, you know, she saw the final result and she loved it," he said.

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Welcome to Rappler, a social news network where stories inspire community engagement and digitally fuelled actions for social change. Rappler comes from the root words "rap" (to discuss) + "ripple" (to make waves).